British intelligence agencies are "very close" to identifying the apparently English jihadist who beheaded an American journalist, the UK's ambassador to the United States has said.

MI5 and MI6 have been piecing together intelligence and evidence from the horrific video footage of the brutal murder to track down the identity of the Islamic State (IS) killer who has been dubbed "jihadi John".

Ambassador Sir Peter Westmacott told US news network CNN that Britain is on the brink of establishing who he is.

"We are very close to identifying who this guy is," he told the State of the Union programme.

Foreign Office and Home Office officials refused to comment on the claims.

"We do not comment on security matters," a Foreign Office spokesman said.

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